Relatively low concentrations of water-soluble, organic scale inhibitors are known to reduce the rate of scale formation in and around the bottom of a producing well. Scales are slightly soluble inorganic salts, such as calcium or barium sulfates or calcium carbonate, etc. The mechanism of the inhibition is probably a combination of sequestering or complexing of multivalent cations and an absorption of inhibitor onto solid surfaces such as the rapid growth planes of newly formed crystalites. Although a wide variety of materials inhibit crystal growth, the compounds most commonly used in well treatments are organic phosphates or phosphonates or adducts of acrylic acid or the like. Where supersaturated, or scale-forming, waters are encountered in an oil production operation, scale inhibitors are commonly injected or "squeezed" into the reservoir formation. The squeezing amounts to injecting the inhibitior and, usually, overflushing the treated zone with water. The well is placed back on production and the entrainment of the inhibitor in the produced water protects the wellbore and downhole equipment from scale build-up.
The squeezing is a convenient way to apply an inhibitor since there is no injection equipment to install or maintain. It is known to be generally desirable that a scale inhibitor be produced in low concentrations within the produced water, such as from about 25 to 100 parts per million. But, this goal is almost never achieved. Generally, most of the injected inhibitor is quickly produced. Various attempts to delay the rate of its production, such as causing it to be adsorbed on the rock surfaces, are only partially successful. And, this has also been true of prior attempts to induce the precipitation within the reservoir of slightly soluble multivalent cation salts of the inhibitor.
For example, U.S. Pat. No. 3,483,925 mentions the problems of obtaining a gradual and uniform return of a squeezed-in inhibitor and proposes the injection of alternating slugs of inhibitor solution and a polymer-thickened solution. U.S. Pat. No. 3,633,672 proposes injecting an inhibitor which forms multivalent cation salts which are only slightly soluble in substantially neutral or alkaline water with both the inhibitor and a compound containing multivalent cations dissolved in an acidic aqueous liquid, so that the relatively insoluble salt of the inhibitor is precipitated when the acid is neutralized within the reservoir. U.S. Pat. No. 3,704,750 suggests injecting a strongly acidic solution of a monovalent cation salt of polyacrylic acid or amide and a salt of a multivalent cation, to cause a similar precipitation of an inhibitor of low solubility when the acid is neutralized within the reservoir. U.S. Pat. No. 3,782,469 proposes that an inhibitor be adsorbed on fracture-propping grains which are then emplaced within a fracture in the reservoir in order to provide a reliably coated source of surfaces form which the inhibitor is desorbed.
Our patent application Ser. No. 150,382, filed May 16, 1980 now U.S. Pat. No. 4,357,248, discloses that the scaling of a producing well can be inhibited by injecting into the reservoir a self-reacting alkaline aqueous solution of a pH-lowering reactant, a compound containing scale-inhibiting anions and a compound containing multivalent cations, which solution subsequently precipitates a slightly soluble scale inhibitor within the reservoir. The disclosures of that application are incorporated herein by cross-reference.